Tools of the type generally mentioned above are used for chip removing or cutting machining of work pieces of metal or the like materials, e.g. composites. A usual machining method is turning, above all in the form of grooving or parting operations, during which the cutting insert is fed radially in a work piece rotating on a center axis while forming a circumferential groove in the same. In grooving, the cutting insert is inserted to a moderate depth in the work piece, while parting requires that the cutting insert is inserted to the vicinity of the center axis. However, the tool may also be mounted in rotatable milling cutters having the purpose of providing straight slots in, for instance, a flat surface of a work piece. In both cases, however, it is required that the blade serving as a holder for the cutting insert has a thickness that is less than the width of a front main cutting edge of the cutting insert, which determines the width of the groove, because otherwise the blade would not clear from the generated surfaces that delimit the groove. As a consequence of their practical application, the tools in question are commonly denominated “parting tools”.
In this connection, it should be mentioned that the replaceable cutting insert is usually manufactured from cemented carbide or some equivalent material having large resistance to wear, while the holding blade is manufactured from steel of a suitable equality. The last-mentioned material has—contrary to cemented carbide—a certain inherent elasticity, which can be utilized to clamp the cutting insert in the seat of the blade.
Parting tools of the kind in question have for a long time been the subject of a progressive development, which has resulted in a large number of construction solutions, several of which make use of self-tensioning clamping or tightening fingers (see, e.g., U.S. Pat. Nos. 5,803,675, 6,241,429, 5,921,724, and 6,579,044). In such tools, the inherent elasticity of the steel of the blade is utilized by holding, in the active state of the tool, the cutting insert resiliently pressed against the bottom support, i.e., without the help of any particular lock member. In order to make the clamping finger self-tensioning, the seat is given an undersize in relation to the thickness of the cutting insert, while an oversize is established by the clamping finger being forced to turn out to an outer position, in which the cutting insert clears. In practice, the undersize and oversize, respectively, may be very moderate and still guarantee a rigid fixation of the cutting insert as well as smooth insert replacements. Thus, in practice, the undersize may be within the range of 0.1-0.2 mm, while the oversize does not need to be more than some hundredths of a millimeter. As a consequence of the tools not relying on particular lock members, such as screws, the retention of the cutting insert becomes reliable, e.g. by the fact that they do not become sensitive to vibrations.
The design of the present tools with self-tensioning clamping fingers, involves, however, a delicate balance between several, most often conflicting demands and desires. On one hand, the spring force of the clamping finger should be sufficiently large to reliably clamp the cutting insert in an exact, predetermined position, but, on the other hand, the spring force must not be so great that the replacement of the cutting insert is too difficult. Furthermore, it is desirable that the holder blade has a long service life. For this reason, the clamping finger should maintain its spring force also after multiple insert replacements. Thus, in practice, at least 80 to 100 replacements should be possible without the spring force being lost. Neither should the clamping finger, in connection with insert replacements, be subject to such a great turning-out force that the yield strength of the steel is exceeded, because then the deformation of the steel may become plastic rather than elastic.
Another factor is the so-called weak section of the blade, i.e., the section along the blade that has the smallest moment of inertia and that therefore most easily is deformed under load by the cutting forces. If the material of the blade would be reduced to too high an extent in the weak section, e.g. as a consequence of an incorrectly dimensioned and/or misplaced hole, the weak section will be dangerously weak. Neither should the material portion in which the steel is deformed to establish the elastic tightening force be too weak (=poor tightening force) or too strong (=excessive deflection force). In other words, not only the seat and the slit/keyhole, but also a possible rear slit for a material-weakening hole, should have an optimized (i.e., neither too large nor too small) area, as well as an optimized location in the blade. A further—and for the user an essential desire—is that the key necessary for insert replacements, and the interacting keyhole, should be of such a nature that the opening and locking, respectively, positions of the key, should be possible to be perceived in a tactile and/or auditive way, rather than visual. Thus, in practice, in the present environment, it is difficult or impossible for the operator to see with the naked eye whether the key has been rotated e.g. 90° between opening and locking positions, in particular if the environment of the tool is occupied by hiding objects.
U.S. Pat. No. 5,803,675 discloses a tool of the initially generally mentioned kind, more precisely in the form of a turning tool having a self-tensioning clamping finger, which can be turned out toward an opening position by means of a key having an eccentric body, which is insertable into a keyhole in the blade. The eccentric body of the key has a markedly elongate or rectangle-like cross-sectional shape (also described as “elliptical” in the publication), which is defined by two flat and parallel no long sides and two short sides having a convex shape. In the keyhole, an upper recess is included, which is delimited by a front, concave sliding surface as well as a central, concave intermediate surface having a smaller radius of curvature, which extends between two sharp edges. When the eccentric body of the key is rotated 90° from an initial position, one of the two convex contact surfaces will be applied against the sharp edges thereof, more precisely with the purpose of retaining the key in an opening state.
One of several disadvantages of the tool known by U.S. Pat. No. 5,803,675 is that the interaction between the key and the keyhole is inferior. Thus, the elongate cross-sectional shape of the eccentric body of the key is determined by a major axis, which is considerably greater than the minor axis thereof. In the exemplified geometry of the keyhole and the eccentric body, respectively, this means that sufficient deflection of the clamping finger becomes very force-consuming (if even possible). In other words, there is an obvious risk of the deformation of the bending zone of the clamping finger will be plastic rather than elastic. Another disadvantage is due to the sharp edges on both sides of the central intermediate surface in the upper recess of the keyhole. When the eccentric body is rotated repeated times between opening and locking positions, these edges lead to a quick wear-out of not only the abutting convex surface on the eccentric body, but also of the proper edges. Therefore, if the dimensions of the eccentric body would—hypothetically—be modified, e.g. by the shortening of said major axis, in order to render a realistic oversize upon opening (less than 1% or 0.05-0.1 mm), the sharp edges in the upper recess of the keyhole will rapidly alter the small dimensional differences that are required to retain a moderate over or opening measure during many insert replacements. In other words, the interacting blade of the tool and the key get a limited service life. In this connection, it should be mentioned that the market requires at least 80 insert replacements without substantially impaired function.